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I have stock 275/55/20 (32") tires on a 2015 3.5 EB 2WD Lariat. Considering putting on 275/65/20 (34") or 285/65/20 (34.5") tires. Been reading here and found out gear ratio is a factor in this decision. My door sticker says L5 which is "3.15 ratio, electronic locking differential". This chart puts my stock 32" tire and 3.15 ratio nowhere near anything "good", it's simply off the chart. Never mind if I go to a bigger wheel.. My truck performs impressively in both gas mileage and power compared to other vehicles I've driven. Am I missing something?
Last edited by Pullsmyboat; 01-26-2019 at 02:28 PM.
i've never heard of a 3.15 RAR on an f150. anyway you definitely need to regear if you are getting any tires bigger than your current ones. if you are getting 34's i would regear to at least 3.73's. 4.10's would be even better
If you're pulling your boat you may want to keep the smaller tires to maintain the torque you can apply to the ground. If you're not struggling to launch with the boat up the ramp then maybe it won't be a big deal. You'll see a mileage hit but it'll look pretty sweet.
The low end torque of the turbo charged 3.5 engine will help make up for the power loss of having 3.15 gears. That chart was developed with non-turbo engines in mind. The numbers listed are the engine rpm. My 5.0 V8 performs best running about 1000 rpm's faster than the 3.5 turbo needs. So I need lower gears to keep rpm's and power up.
That said, I would be hesitant to go to a 34" tire with those gears. Even 32" tires are borderline if you tow heavy and/or in mountains. Seeing that you are in south Florida you may not notice that much power loss. Drive that truck to the mountains and get over 6000' elevation and on a steep incline and you may find your truck under powered. Re-gearing is an expensive undertaking, and not worth having 34" tires to me.
Gearing is about mechanical advantage. By relying on 'power' to turn the heavier/larger tires you are subjecting the engine and transmission to unnecessary wear & strain. Additionally, you'll get better avg mpg if geared properly.
850hp trophy trucks use 5.13s to spin 39s - nobody is immune to physics.
Edit: just saw the above questions
4.10 is baseline for 34-35" tires. 6spd or 10spd doesn't matter.
Rule of thumb is to figure out the baseline ratio, then take into account weight and rolling resistance. Ex - if 4.10s are baseline for a relatively tame set of 35" BFG A/Ts, but you are installing something much heavier and or an aggressive tread - consider bumping to 4.56 to compensate. Rolling resistance and rotational mass have a negative seat of the pants impact, even if the gearing is 'right'.
Using calculators are tricky because if your truck was castrated from the factory, it'll still be castrated after the calc.
Look up my gearing tech thread, I go over everything in there (Im on my phone).
Thanks for the suggestions. All this is new to me. I've been reading and now have a better understanding. This video was helpful to put things in perspective.
01-26-2019, 02:10 PM
Gear ratio tire size question?
